The invention relates to a method of drying a fiber web by an apparatus comprising two endless bands that are impermeable to air; first turning rolls, around which the first band is arranged to turn; and second turning rolls, around which the second band is arranged to turn; the first band and the second band being arranged to run part of the way in parallel such that they define a drying zone between them; the first band being heated and the second band being cooled by a cooling chamber; and the fiber web and at least one felt or wire being arranged to run between the bands so that the fiber web is in contact with the first, heated band and that the felt or wire is between the fiber web and the second, cooled band.
The invention also relates to an apparatus for drying a fiber web, the apparatus comprising two endless bands that are impermeable to air; first turning rolls, around which the first band is arranged to turn; and second turning rolls, around which the second band is arranged to turn; the first band and the second band being arranged to run part of the way in parallel such that they define a drying zone between them; the first band being arranged to be heated by heating means and the second band being arranged to be cooled by a cooling chamber; the fiber web and at least one felt or wire being arranged to run between the bands so that the fiber web is in contact with the first, heated band and that the felt or wire is between the fiber web and the second, cooled band.
Many patent publications, such as WO 96/11300 and U.S. Pat. No. 4,461,095, teach the drying of a fiber web between two parallel metal bands moving in the same direction such that the fiber web is in contact with the heated metal band and that there is a wire between the fiber web and the second, cooled metal band, whereby the steam separated from the fiber web by heating condenses to the wire by the effect of the cold metal band. The basic idea is that two endless metal bands are arranged to turn around turning rolls and that against the inner surface of the loops formed by the bands are provided pressure chambers containing hot steam and water, respectively, such that the pressure produced presses the hot and cold bands, respectively, against the fiber web and the wire running between them. The bands located between the pressure chambers define one side of the pressure chambers by means of seals, whereby the steam and the water can directly affect the bands. The operation of the apparatus is fully known per se and has been disclosed, for example, in the above patent publications, which are incorporated herein by reference.
Finnish Patent 76 856 discloses a similar solution as described above: the web to be dried is transferred on a felt between endless bands running around. The band in contact with the web is heated by a steam chamber and the band in contact with the felt is supported and cooled by support and cooling means at the steam chamber. A hydrostatic plate is used as the support and cooling means, the hydrostatic plate having several adjacent pressure pockets and return channels separated from one another by means of ridges. Water is supplied to the pressure pockets so that it discharges over the ridges into the return channels. The solution works superbly, but as the speed of the paper machines grows, the solution of controlling the friction and the pressure on the water table needs to be improved.
The object of the present invention is to provide a method and an apparatus for quick and efficient drying of a fiber web.
The method of the invention is characterized in that the cooling chamber comprises at least one cooling element, which in turn comprises at least one inlet chamber and at least one outlet chamber; that the pressure of the cooling element is balanced by pressure balancing means; and that the second band is provided with a bearing at the cooling element.
The apparatus of the invention is characterized in that the cooling chamber comprises at least one cooling element, which in turn comprises at least one inlet chamber and at least one outlet chamber, and that the cooling element also comprises at least one bearing and pressure balancing means.
The essential idea of the invention is that the fiber web is dried between the heated and the cooled band by cooling the band by cooling means that comprise at least one cooling element. The cooling element comprises at least one inlet chamber and at least one outlet chamber. A cooling medium is supplied via the inlet chamber to the cooling element, from which it passes over a ridge formed by the walls of the outlet chamber to the outlet chamber, and is subsequently discharged therefrom. The pressure of the cooling element is balanced by the pressure balancing means. In a preferred embodiment, the pressure is balanced by supplying the cooling medium along pressure balancing pipes from the end of the cooling element toward the beginning, seen in the travel direction of the cooled band. In another preferred embodiment, the cooling element comprises slide shoes, which provide hydrodynamic bearings for the cooled band with the help of the cooling medium. In a third preferred embodiment, support pins are arranged in the cooling element for the cooled band.
The advantage of the invention is that the balancing of the pressure in the cooling element prevents the cooling medium from passing toward the end of the apparatus and from being discharged from the apparatus at the seal of the rear end. Since the pressure does not vary, the cooled band and the bearing of the band can also be better controlled. On account of the slide shoes that provide the hydrodynamic bearing, the providing of the cooled band with a bearing is extremely successful. The support pins prevent the occurrence of plastic changes in the bands.